I came across a test by Hot Rod magazine in which an 11 hp. gain was made by simply swapping the coil packs and plugs in a 5.7 Hemi.

I found this horsepower gain surprising. Although the new coil packs produced 40,000 volts under load, as opposed to 25,000 volts for the stock ones, it is common knowledge that aftermarket ignition systems, high voltage coils, etc. do not result in horsepower gains for low rpm stock engines. Where they are needed is in 8-9,000 rpm race engines.

Furthermore, a good spark plug firing in a normal engine will discharge around 8k volts or so, which is way less than a coil that has the ability to put out 40, 50, or even 70k volts. The spark will jump the plug gap when it can and this is usually under the 10k volts mark.

Ignition system improvements have been the best area for fake claims as it is easy to change the components, plugs and coils, but hard to actually measure the performance gain if any for that change.
The FTC reached a settlement with Split Fire for such claims:https://www.ftc.gov/news-events/pre...les-ftc-charges-economy-efficiency-claims-are
Mopar Action's Richard Ehrenberg has also stated many times that if the stock ignition is firing the plugs properly, higher output ignition or multiply pathway plugs will not improve things.
Many people let the ignition go to the point where misfires occur. They then "upgrade" to high performance plugs or coils and claim that there is a power increase when what really happened was a simple tune-up restored original performance. My '68 340 Barracuda never benefited from Accel distributor, cap, rotor, and performance plugs. It turned a 13.40 @ 105.38 with stock components and never improved with aftermarket parts.